HYDRODYNAMIC INVESTIGATION OF A SHALLOW TROPICAL LAKE ENVIRONMENT (LAGUNA LAKE, PHILIPPINES) AND ASSOCIATED IMPLICATIONS FOR EUTROPHIC VULNERABILITY Eugene Herrera1, Kazuo Nadaoka2, Ariel Blanco3, and Emiterio Hernandez4 1 National Hydraulic Research Center, College of Engineering, Institute of Civil Engineering, University of the Philippines, Diliman, Quezon City, Philippines, Tel: (632) 9540243, e-mail: [email protected] 2 Department of Mechanical and Environmental Informatics, Graduate School of Information Science And Engineering, Tokyo Institute of Technology, Tokyo, Japan, Tel: (813) 57342589, e-mail: [email protected] 3 Department of Geodetic Engineering, University of The Philippines, Diliman, Quezon City, Philippines, Tel: (632) 9208924, e-mail: [email protected] 4 Integrated Water Resources Management Division, Laguna Lake Development Authority, Philippines, Tel: (632) 9819948, e-mail: [email protected] Received Date: March 31, 2014 Abstract In this study, a three-dimensional (3D) hydrodynamic model was developed to investigate the water circulation characteristics of a shallow tropical lake environment (Laguna Lake, Philippines) under varying wind stress, watershed river discharge, and sea interaction (Manila Bay, Philippines) to elucidate hydrodynamic implications to eutrophic vulnerability. The analyses were based on field observations and numerical simulations covering long-term periods (dry and wet season) and intensive field measurements. The results demonstrated different circulation patterns and vertical current regimes in time and space that accentuated a thermally stratified lake environment in spite of a shallow water depth (2.5 meter average). In the wet season, current movement is generally toward Pasig River, the lake’s only outlet, with embayment water mass moving in the clockwise direction. The dry season lake flow pattern was characterized by counter-clockwise gyre circulations, Pasig River backflow, and salinity intrusion. Wind forces showed strong influence on lake hydrodynamics due to the complex surrounding topography, steep land-lake thermal difference, regular passage of tropical storms and typhoons, and shifts in the monsoon wind direction. The resulting mixed layer hydrodynamics revealed important implications to the planktonic movement, nutrient recycling, and primary production of the lake. Long term continuous observations and numerical analyses also demonstrated the intrusion of seawater to Laguna Lake that potentially adds to its eutrophic vulnerability with the associated entry of nutrient and microorganism-rich polluted waters from Metro Manila. Keywords: Delft3D, Hydrodynamics, Laguna de Bay, Modeling, Water quality Introduction The fresh waters of the world are collectively experiencing markedly accelerating rates of qualitative and quantitative degradation [1]. The scarce supplies of inland fresh-water are exploited by the rapidly expanding human population, growing industrial economy and extensive urbanization. These factors severely affect water quality and, hence, environmental integrity. The control and reversal of degradation requires proper economic ASEAN Engineering Journal Part C, Vol 4 No 1 (2015 EnvE Special Issue), ISSN 2286-8151 p.48 and social valuation of fresh waters. The first step, ultimately, is a comprehensive understanding of the physical, chemical and metabolic mechanisms of these ecosystems. However, most current literature on general limnology remains temperate-based as tropical limnology only developed in the late 20th century. Limnology in the tropics has only recently developed past the stage of exploration [2, 3]. In particular, only a small fraction of the research has focused on the mechanisms of shallow tropical lakes, despite the limnological significance of these aquatic ecosystems. The ecosystem response to environmental degradation considerably differs between tropical and temperate lakes [4]. The foundation of protective and diagnostic regulations of lakes is based on decades of scientific research on temperate lakes and cannot be readily applied to tropical lakes. Therefore, comprehensive research on tropical limnology is warranted for establishing a scientific foundation for the proper treatment and management of tropical inland ecosystems. Laguna Lake, one of the largest lakes in Southeast Asia, is one of the most important natural water-resource bases in the Philippines. Strategically located at the center of an urban development, Metro Manila, it is the focal point of national and regional development efforts in the agriculture and fishery, water supply and energy sectors [5]. At the same time, however, Laguna Lake is stressed with competing water-users and continued environmental degradation from anthropogenic-based stressors. The dynamic interaction of the lake with Manila Bay through the Pasig River provides salt-water interaction, which makes it ideal for the fishery and aquaculture industries. However, nutrient-rich polluted water also discharges from Metro Manila and surrounding coastal provinces to the lake. As a result, massive fish kills have been a regular occurrence [5, 6] and has become a management priority. The pressures of both growing user-demand and declining water quality have increasingly stressed the value of scientific knowledge on understanding the mechanisms of the lake environment for the optimal and sustainable use of its water resources. A need for hydrodynamic and water quality research of Laguna Lake is warranted to generate reliable information for improving lake conservation and management programs. This study investigates the hydrodynamic features of Laguna Lake with the use of intensive and extensive field surveys and high resolution numerical modeling analyses. The study primarily aims to clarify the circulation and transport features of a tropical lake ecosystem and provide information on water quality dynamics affecting critical ecosystem conditions. The implications for eutrophic vulnerability of the lake were elucidated based on the analyses of lake hydrodynamics. Climatic, hydrodynamic and bio-chemical parameters were measured using various data-logging sensors and water samplings. Long- term continuous measurements were established to capture seasonal and annual variations of environmental variables. A three-dimensional hydrodynamic model was set-up to evaluate the circulation and mass transport characteristics of the lake. The results presented in this paper could potentially provide insight into the physical-biochemical dynamics and functioning of shallow tropical lakes in urban environments in the Southeast Asian region and may correspondingly fill-in existing knowledge gaps in tropical limnology. Materials and Methods Laguna Lake Physical Environment Laguna Lake (located at 14o11’-14o33’ N, 121o03’-121o29’ E) is the largest lake in the Philippines with a surface area of 900 km2 (Figure 1). The lake is approximately maple leaf-shaped with four distinct lobes, namely, ‘West Bay’, ‘Central Bay’, ‘East Bay’, and ‘South Bay’, delineated by a total shoreline length of 285 km. The shallow average depth of 2.5 m ASEAN Engineering Journal Part C, Vol 4 No 1 (2015 EnvE Special Issue), ISSN 2286-8151 p.49 accounts for its characteristic turbidity [5] with a Secchi disc transparency reading of less than 25 cm for low salinity intrusion years. The water level in the lake oscillates with an annual amplitude of approximately two meters. Laguna Lake has natural brackish waters due to its interaction with Manila Bay through the tidally affected Pasig River. The 27-km Pasig River serves as the only outlet of the lake. In the dry season, when the lake level is lower than Manila Bay, and when there is sufficient tidal fluctuation, the flow reverses. During the backflow of the Pasig River, the river discharges nutrient-rich water and the lake experiences an intrusion of polluted sea water, promoting fishery and aquaculture as the lake’s most dominant economic function. The lake annually contributes approximately 85,000 metric tons of fish [7,8] to the fish supply of Metro Manila and nearby provinces. This accounts for approximately 40% of the total fish production through aquaculture in the Philippines. Fish pen belts and fish cage belts at specified locations in the lake, cover a total area of 100 km2 and 50 km2, respectively [4]. Silt-sized particles predominate in the surface sediments of Laguna Lake. However, clay-sized particles are abundant in West Bay likely due to the high yield of fine-grained sediments of the Pasig River and to the common occurrence of seawater intrusion-induced flocculation [9]. Figure 1. Location of Laguna Lake, Philippi nes. Shown also are the relative locations of Manila Bay, Pasig River, Napindan Channel, West Bay, Central Bay, East Bay, South Bay, and th e Lake watershed Field Surveys and Sampling Procedures The continuous intensive monitoring of meteorological, hydrodynamic, and water quality parameters was facilitated through a monitoring platform constructed at the northwestern lobe of the lake (Figure 1). The platform was established as a part of the objectives for the collaborative data monitoring and research of the Laguna Lake environment between the Tokyo Institute of Technology, Japan (Nadaoka Laboratory), and the Department of Environment and Natural Resources, Philippines (Laguna Lake Development Authority). ASEAN Engineering Journal Part C, Vol 4 No 1 (2015 EnvE Special Issue), ISSN 2286-8151 p.50 The platform was configured to have both weather and